This invention relates to detectors for ionizing radiation, particularly x-ray detectors used in computed axial tomography systems for medical diagnosis. Such detectors must detect x-ray photons efficiently and with a high degree of spatial resolution. In some computed axial tomography systems, the x-ray source is pulsed and the pulse repetition rate can be limited by the recovery time of the x-ray detectors. Optimized detectors for use in such systems will thus have fast recovery time, high sensitivity, and fine spatial resolution. In multicell detectors, the cells should each have identical and stable detecting characteristics.
Multicell detector arrays are shown in U.S. Pat. No. 4,119,853, issued to Shelley, et al. on Oct. 10, 1978, and U.S. Pat. No. 4,276,476, issued to Cotic on June 30, 1981. The detectors in the cited patents comprise a multiplicity of adjacent, slightly spaced apart electrode plates standing edgewise within a pressure vessel containing an ionizable gas. Ionization events can take place in the gas filled gaps between adjacent plates when ionizing radiation passes through a window in the pressure vessel and enters the gaps. Alternate electrode plates in the array are connected together and to a common potential source to provide bias electrodes. The remaining electrodes, called signal electrodes, have individual electrical leads connected to a data processing unit (typically a digital computer) to allow the potential between each signal electrode and the most nearly adjacent bias electrodes to be measured. The potential between any signal electrode and the bias electrodes is proportional to the instantaneous intensity of x-radiation in the space between adjacent bias electrodes of the array.
The electrode plates are positioned with their front edges in equally spaced relation to an x-ray transmissive window formed in the front wall of the chamber, and are disposed along regularly spaced radial lines extending from the source of radiation. To maintain this spacing and disposition the top and bottom edges of the plates are received in registered pairs of radial grooves formed in a pair of electrically insulating ceramic substrates, which in turn are bonded to the facing surfaces of a pair of curved frame members, typically stainless steel bars. End blocks secured between the bars at each end of the array complete the electrode array assembly. The end blocks are supported within the chamber to locate the array therein.
The described multicell detectors can be susceptible to high frequency mechanical vibrations known as microphonics. The electrode plates are made of extremely thin metal and are maintained close together and with a relatively large potential difference between them. Microphonics transmitted through the gas chamber to the electrode array affect the capacitance between adjacent electrodes and can introduce spurious signals which change the x-ray intensity measurements. In severe cases, microphonic noise can be comparable in intensity to the x-ray induced signal, thus significantly reducing the accuracy of the reconstructed image.
The cited art shows a detector array end block or post biased toward the front and bottom walls of the chamber by cantilevered finger springs. The bias is resisted by resilient spacers interposed between the array and the front and bottom walls of the chamber, providing a floating array which is securely supported within the chamber. Such a structure has considerably reduced the effects of microphonics on the detector, but further improvement is possible. The cantilevered springs for biasing the array toward the front wall of the chamber can exert a turning moment on the array. In addition, metal particles are sometimes scraped by the springs from the rear wall of the chamber during insertion of the prior art array in the chamber, and can seriously interfere with detector performance.